1. Field of the Invention
The present invention relates to the utilization of a post vulcanized rubber crumb combined with asphaltites to modify the physical performance of asphalt. More particularly, the invention relates to additives to the rubber crumb and asphaltite combination.
2. Description of the Prior Art
The utilization of post vulcanized rubber crumb, derived from the processing of scrap automobile and truck tires, to modify asphalt pavements has been common for a number of years. The general terminology used by the Federal Highway Administration, xe2x80x9cFHWAxe2x80x9d, for crumb rubber modifiers is xe2x80x9cCRM.xe2x80x9d Crumb rubber differs from other elastomers or polymers, used to modify asphalt, in that crumb rubber is vulcanized or crosslinked (the process forms irreversible chemical bonds) prior to use. xe2x80x9cCRMxe2x80x9d is incorporated into asphalt pavements by either a wet or dry process.
The xe2x80x9cwetxe2x80x9d process combines the rubber crumb with the asphalt binder under heat and agitation until reacted or digested and dispersed, prior to the addition of aggregates. The xe2x80x9cdryxe2x80x9d process typically specifies a high Natural Rubber, xe2x80x9cNR,xe2x80x9d content in the crumb rubber which often eliminates scrap automobile tires from use. Of the two processes the xe2x80x9cwetxe2x80x9d method is in wider use.
The xe2x80x9cdryxe2x80x9d process has produced pavements that tend to ravel and seems to have lost favor with highway engineers. The xe2x80x9cwetxe2x80x9d process is dependent on a heat induced reaction (digestion) of the crumb rubber into the asphalt. The crumb rubber particle surface area, grade of asphalt, temperature, and even rubber particle dispersion effect the distinct characteristics of resultant xe2x80x9cCRMxe2x80x9d pavement.
The xe2x80x9cwetxe2x80x9d process can be divided into two prevalent methods. The xe2x80x9cArizona Designxe2x80x9d using a 10-mesh and smaller rubber particle normally of high xe2x80x9cNRxe2x80x9d content, which has been ambiently processed. The xe2x80x9cArizona Designxe2x80x9d batch blends the crumb rubber with asphalt, at 375-425xc2x0 F., until a gel of required viscosity is formed (typically one hour). The gel has a pot life determined by a lessening of viscosity as the rubber particles decompose.
The newer xe2x80x9cwetxe2x80x9d method is the xe2x80x9cUltraFine Floridaxe2x80x9d process which uses 80 mesh and smaller rubber particles of high xe2x80x9cNRxe2x80x9d content, typically processed by wet grinding. The xe2x80x9cUltraFinexe2x80x9d process tends to use lesser content of crumb rubber and may be blended at the asphalt terminal.
The purpose of adding crumb rubber to asphalt in pavement applications is to enhance the asphalt binder""s elastomeric properties thereby improving resistance to stress cracking, thermal cracking, reflective cracking, rutting and general pavement durability. In general xe2x80x9cCRMxe2x80x9d pavements have performed poorly in pavement stability and have required a higher asphalt binder content. Even though use is motivated by political mandates, xe2x80x9cISTEAxe2x80x9d public law (102-240) Dec. 18, 1991, xe2x80x9cCRMxe2x80x9d pavements have had limited industry success. This lack of success relates directly to cost, performance and difficulty of use.
Gilsonite (unintaite), Glance Pitch and Grahamite are natural occurring hydrocarbon substances characterized by a high softening point (above 110xc2x0 C.) in the class known as asphaltites. They are mined much like other minerals and sold essentially in their native state. They are fully compatible with asphalt and have long been known as asphalt hardeners and reinforcing agents. Gilsonite is currently sold all over the world as an asphalt modifier in the form of a dry bulk solid granular powder.
Gilsonite""s benefits to asphalt pavements include increased stability, resistance to deformations problems such as rutting and shoving, resistance to water striping and increased load bearing ability. Gilsonite functions by making the pavements harder, stronger and increases asphalt""s adhesion to aggregates. It is generally regarded that Gilsonite reduces pavements"" low temperature properties making them susceptible to thermal cracking.
Gilsonite melted into hot asphalt will reduce penetration and increase viscosity of the asphalt binder. Gilsonite may also be mixed with aggregate prior to combining with the asphalt binder. Gilsonite modified asphalt pavements have been particularly successfully in highly stressed traffic areas. Gilsonite, as the majority constituent, has been combined with virgin polymers such as styrene-butadiene-styrene (SBS) and Ethyl Vinyl Acetate (EVA). Gilsonite modified asphalt binders generally do not increase asphalt binder content requirement in pavement mixtures.
Performance grading of asphalt binders and pavement mixtures became a reality with the conclusion of the, xe2x80x9cFHWAxe2x80x9d 50 million dollar, Strategic Highway Research Program, xe2x80x9cSHRPxe2x80x9d, in March of 1993. xe2x80x9cSHRPxe2x80x9d developed new asphalt binder specifications and test criteria based on the engineering properties related to pavement performance. The new emphasis is on low temperature performance of aged binder materials. Low temperature flexibility of aged asphalt binders became significant. Performance Grade xe2x80x9cPG,xe2x80x9d asphalt is based on the predicted temperature performance both high and low of asphalt binders. Neither Gilsonite nor post vulcanized crumb rubber have performed well under xe2x80x9cSHRPxe2x80x9d evaluations. xe2x80x9cSHRPxe2x80x9d specifications will cause increased demand for modified asphalt as state departments of transportation adopt the xe2x80x9cPGxe2x80x9d specifications.
It is well-known in the art to use tall oil with ground rubber waste for reuse as rubber. See xe2x80x9cGround Rubber Wastexe2x80x94A Supplementary Raw Material for the Rubber Industryxe2x80x9d issued by Kahl and Co.; U.S. Pat. No. 4,481,335, issued Nov. 6, 1984 to Stark, Jr. entitled xe2x80x9cRubber Composition and Methodxe2x80x9d; U.S. Pat. No. 3,873,482, issued Mar. 25, 1975 to Severson et al, entitled xe2x80x9cPyrolyzed Tall Oil Products as Synthetic Rubber Tackifiersxe2x80x9d; U.S. Pat. No. 4,895,911, issued Jan. 23, 1990 to Mowdood et al, entitled xe2x80x9cTall Oil Fatty Acid Mixture in Rubberxe2x80x9d; U.S. Pat. No. 4,792,589, issued Dec. 20, 1988 to Colvin et al, entitled xe2x80x9cRubber Vulcanization Agents of Sulfur and Olefinxe2x80x9d; and U.S. Pat. No. 4,244,841, issued Jan. 13, 1981 to Frankland, entitled xe2x80x9cMethod for Recycling Rubber and Recycled Rubber Productxe2x80x9d. Generally for the area of ground polymer elastomer operation, see U.S. Pat. No. 4,771,110, issued Sep. 13, 1988 to Bouman et al, entitled xe2x80x9cPolymeric Materials Having Controlled Physical Properties and Purposes for Obtaining Thesexe2x80x9d; and for rubber discussions see U.S. Pat. No. 3,544,492, issued Dec. 1, 1970, to Taylor et al, entitled xe2x80x9cSulfur Containing Curing Agentsxe2x80x9d; and xe2x80x9cOrganic Chemistryxe2x80x9d by Fieser and Fieser printed 1944 by D.C. Heath and Co. Boston, pages 346 and 347. See also U.S. Pat. No. 2,082,607, issued Sep. 1, 1936 to Cowdery, entitled xe2x80x9cRubber Compounding Material and Method of Usingxe2x80x9d and U.S. Pat. No. 2,133,690, issued Oct. 13, 1938 to Epstein, entitled xe2x80x9cRubber and Method of Making the Samexe2x80x9d.
The present invention is a recipe and method for combining Gilsonite or other asphaltites with post vulcanized (cured) elastomers which have been further processed by ambient or cryogenic or wet grinding into powder or granules. The resulting mixture is designed to modify the physical performance characteristics of asphalt or bitumen (technically xe2x80x9cbitumenxe2x80x9d is used to denominate raw material heavy portions of crude oil, but in the Gilsonite industry xe2x80x9casphaltxe2x80x9d and xe2x80x9cbitumenxe2x80x9d are used interchangeably) used in industrial, consumer and road pavement applications. The invention further discloses the recipe and method to create a Dry Liquid Concentrate, xe2x80x9cDLCxe2x80x9d comprised of post vulcanized (cured) elastomers processed by ambient or cryogenic grinding into powder or granules and tall oil, tall oil heads, tall oil pitches, residues of tall oil production and other fatty acids which enhances the post vulcanized rubber powder or granules effectiveness in the invention.
The preferred asphaltites are characterized by their high softening points above 110xc2x0 C. and include Gilsonite (uintaite), Glance Pitch and Grahamite.
Post vulcanized crosslinked elastomer(s) which have been further processed by ambient or cryogenic or wet grinding into cured rubber granules or powder forms a primary component of the mixture. The cured rubber particles used are of natural or synthetic rubber, or combination thereof, which has been substantially vulcanized or cured, as in the manufacture of automobile or truck tires. Scrap tires, including but not limited to, automobile and truck tires constitute a primary source of available, useful cured rubber particles. With respect to scrap tires as a source of cured rubber particles the mixture is equally effective with crumb rubber generated either from the side wall or tread of scrap automobile or truck tire carcasses. Common rubbers useful to the invention include, but are not limited to: NR, SBR, isoprene, neoprene, nitrile, butyl and ethylene-propylene dien rubbers. There is no need to separate the rubbers by polymer content.
The preferred Dry Liquid Concentrate, xe2x80x9cDLCxe2x80x9d, is a homogeneous or unitofrm blend or combination or mixture of tall oil, tall oil heads, tall oil pitches, residues of tall oil production, or other fatty acids, post vulcanized elastomer(s) which have been processed into granules or powder (i.e., crumb rubber) and converting vulcanized crumb rubber into the DLC by absorption by the crumb rubber of such, for example, tall oil agents or other fatty acids, and then by mixing with Gilsonite or other asphaltites becomes an asphaltite reinforced DLC. The preferred Dry Liquid Concentrate, xe2x80x9cDLCxe2x80x9d, is a homogeneous or uniform blend or combination or mixture of tall oil, tall oil heads, tall oil pitches, residues of tall oil production, or other fatty acids, and post-vulcanized elastomer(s) which have been processed into granules or powder (i.e. crumb rubber) and converts vulcanized crumb rubber in the DLC by absorption by the crumb rubber of such, for example tall oil agents or other fatty acids. The xe2x80x9cDLCxe2x80x9d enhances the performance capabilities of this invention by accelerating the heat blending cycle of the crumb rubber with asphalt or bitumen, thereby reducing the cost of blending equipment, reducing required energy and allowing the invention to be employed on either the aggregate or asphalt side of a pavement mixture.
The mixtures disclosed in this invention substantially modify the temperature performance, both high and low, as well as increase the durability of asphalt pavement mixtures.